1. Field of the Invention
The present invention relates to a circuit board on which electronic parts can be mounted. More particularly, the present invention relates to a circuit board in which wiring layers are electronically connected with each other by a via conductor.
2. Description of the Related Art
The circuit boards on which electronic parts, such as semiconductor devices, are mounted are recently increasingly multilayered in response to the increase in density of circuit arrangement. Generally, wiring layers in a circuit board are electrically insulated from each other by an intervening insulating layer and electrically connected with each other by a via conductor that penetrates the insulating layer at a predetermined position.
More specifically, when a high-density multilayer printed circuit board is manufactured using a glass epoxy substrate made of glass fiber impregnated with epoxy resin, blind via holes are formed by a CO2 laser processing. Reference (1) listed in the following Related Art List discloses a typical example of a circuit board with such via holes formed therein. With a circuit board as described in Reference (1), the resin layer therein contains glass fibers to give strength thereto, and parts of the glass fibers projecting from the side wall of the via hole form an anchor structure with a plated layer formed in the via hole.
Related Art List
(1) Japanese Patent Application Laid-Open No. 2004-288795.
With such a conventional circuit board, laser processing leaves the sections or end faces of glass fiber exposed at the side wall surface of the via hole. When these glass fibers are projecting toward the center of the via hole from the side walls thereof, formation of a plated layer on the side wall of the via hole creates abnormal growths of plating at the section or end face of glass fiber, with the result that the portions of the sections or end faces of glass fiber project into the via hole. These abnormal growths of plating will make it difficult to embed photo solder resist (PSR) fully in the via hole, thus reducing the connection reliability of the via.